Flow visualization also presents a unique opportunity to quantify directly the effect of complex flows on fish behaviour. Simultaneous visualization of a cylinder wake and Ka´rma´n gait kinematics revealed that fish move with the lateral component of the sinusoidal flow in a vortex street, slaloming between vortices rather than swimming through each vortex centre. Lateral body displacement can be described using an arbitrary phase relationship relative to the position of the drifting vortices. For example, when a vortex drifts down to the region of the head , the head and the vortex have a phase relationship of approximately 1008, indicating that the head is moving laterally away from the vortex. When a vortex drifts down to the body’s COM, the COM has reached its lateral-most excursion from that vortex and is starting
to move back towards the midline of the wake. At all body points posterior to the COM, the phase relationship is greater than 1808. Thus, when a vortex drifts to the tail , the phase relationship between vortex and body is approximately 2308, so the tail is moving laterally towards the vortex. The phase relationships between different points along the body
and vortices are consistent with the observation that the body is being translated by the lateral component of the
flow established by the preceding vortex. The ability to Ka´rma´n gait relies predominantly on encountering vortical flows of the proper strength and size, which is determined by the flow velocity and ratio of cylinder diameter to fish length. Altering this ratio can substantially affect the preference of fish to associate with cylinders. From a hydrodynamic viewpoint, it stands to reason that fish in natural environments will distribute themselves accordingly among different size structures, where structural preference has been correlated to body length in the field .
Flow visualization also presents a unique opportunity to quantify directly the effect of complex flows on fish behaviour. Simultaneous visualization of a cylinder wake and Ka´rma´n gait kinematics revealed that fish move with the lateral component of the sinusoidal flow in a vortex street, slaloming between vortices rather than swimming through each vortex centre. Lateral body displacement can be described using an arbitrary phase relationship relative to the position of the drifting vortices. For example, when a vortex drifts down to the region of the head , the head and the vortex have a phase relationship of approximately 1008, indicating that the head is moving laterally away from the vortex. When a vortex drifts down to the body’s COM, the COM has reached its lateral-most excursion from that vortex and is startingto move back towards the midline of the wake. At all body points posterior to the COM, the phase relationship is greater than 1808. Thus, when a vortex drifts to the tail , the phase relationship between vortex and body is approximately 2308, so the tail is moving laterally towards the vortex. The phase relationships between different points along the bodyand vortices are consistent with the observation that the body is being translated by the lateral component of theflow established by the preceding vortex. The ability to Ka´rma´n gait relies predominantly on encountering vortical flows of the proper strength and size, which is determined by the flow velocity and ratio of cylinder diameter to fish length. Altering this ratio can substantially affect the preference of fish to associate with cylinders. From a hydrodynamic viewpoint, it stands to reason that fish in natural environments will distribute themselves accordingly among different size structures, where structural preference has been correlated to body length in the field .
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